In recent years, there is an increasing demand for smaller built-in camera modules for use in mobile phones, laptop computers, electronic tablets, etc. These camera modules use an image sensor to detect light and convert the detected light into a signal that is used to produce an image. Miniaturization of the image sensor in these camera modules requires decreasing the size of the pixel(s) while increasing the number of pixels in order to maintain an acceptable quality in the produced image. However, decreasing the size of the pixel reduces the amount of signals from the image sensor. Further, the miniaturization of a pixel reduces the light volume available to the pixel, and the reduced light volume that enters into a pixel causes a greater degradation of the signal to a point where signal-to-noise ratio (SNR) becomes a problem. The goal is to attain high sensitivity by improving light utilization efficiency.
Some conventional image sensors include a single photoelectric conversion element that detects all colors of light and synthesizes a color image therefrom through color reproduction by interpolation methods. Other conventional image sensors use color filters which absorb wavelengths that are not recognized by the photoelectric conversion element, and the absorbed wavelengths that are not recognized do not contribute to the reproduction of the image. This results in inefficient light utilization which may decrease the resolution the image. In other conventional image sensors, multiple dichroic mirrors are placed in the path of incident light, which results in enhanced color sensing by the image sensor. However, manufacture of these types of image sensors is very expensive. Additionally, the wavelength characteristics of the dichroic mirrors largely depend on the incidence angle of incoming light, which may cause the half wave length in the spectral characteristic to shift by several ten nanometers due to a difference in the incidence angle. The difference in the spectral characteristics is further exaggerated because the dichroic mirrors have different wavelength characteristics, which causes a degradation of the color reproduction. In addition, due to light polarization in the dichroic mirror, the transmission and the reflection characteristics differ greatly due to the presence of parallel (P) waves at the incident surface and perpendicular (S) waves at the incident surface. The presence of these waves causes the color separation characteristics to degrade.
What is needed is an image sensor having greater color sensitivity and improved optical efficiency that is less expensive to manufacture.